Image forming devices such as laser printers, copy machines, and facsimile machines employ electro photographic processes to produce printed images on paper or other print media. In laser printers, a source image is rasterized to form a bitmap image for subsequent rendering to a final printed image. A source image can include a number of elements. Examples include text, line art, and/or continuous tone or graphic elements such as a photo. When a source image is rasterized creating a bitmap image, the various elements are converted to pixel patterns that approximate the source image.
Once a bitmap image is generated from a source image, it can be printed by modulating and scanning a laser beam across a charged surface of a photoconductive drum in a succession of scan lines. Each scan line is divided into pixel areas, and the modulated laser beam causes some pixel areas to be exposed to a light pulse and some not. Where a pixel area is illuminated, the photosensitive drum is discharged so that, when it is subsequently toned, the toner adheres to the discharged areas and is repelled by the still charged areas. The toner adhering to the discharged areas is then transferred and fixed to paper or other print media.
Because of its digital nature, a printed bitmap image does not perfectly reflect the source image. For example, printed image edges that are either not parallel and not perpendicular to the scan direction of the laser can appear stepped or jagged. This is especially noted for text and line art. Because resolution is also limited, a bitmap representation of a graphical element such as a photo can appear degraded.
Various image enhancement techniques have been developed to improve the quality of a printed bitmap image. These enhancement techniques include: edge smoothing, fine line broadening, anti-aliasing, and resolution doubling. In a laser printer for example, one or more techniques can, when implemented, modulate the laser to produce a pixel smaller than a standard pixel size. The techniques may also indicate that the laser output is to be offset from the pixel center. Parameters for a particular image enhancement technique can dictate pixel size and offset position.
In addition to improving the quality of printed images, printer manufacturers are also faced with reducing the cost associated with printing those images. One of the more significant operation costs of a printer is that of imaging material such as toner and ink. Recognizing this, various imaging enhancement techniques have been developed in an effort to conserve imaging material.
The various moving components of an image forming device, especially the photoconductive drum, wear and degrade through use. Use exposes the photoconductive drum to paper and toner particles that can leave scratches on the drum's surface. Scratches reduce the effective resolution of the drum. For example, when the drum is exposed to a laser, a larger than intended area of the drum may be discharged because of a scratch. Use can also cause the photoconductive drum to “thin.” Thinning is caused by ozone which results from the electrical charge used to “erase” the photoconductive drum after each page is printed. The ozone reacts with the surface material of a photoconductive drum causing the drum to thin over time. As it becomes thinner, a photoconductive drum becomes less effective in maintaining a charge differential between those parts of the drum that are exposed to a laser and those parts that are not.
In many image forming devices, the photoconductive drum is integrated in a cartridge that also serves as a reservoir for toner. When the toner is depleted, the cartridge can be removed from the image forming device and replaced. The rated life of a cartridge can be measured by the amount of toner it holds. In other words, the components of the cartridge are designed to, at a minimum, outlast the toner. Photoconductive drums are designed to enable printing for the rated life of the cartridge such that thinning and scratching usually do not affect print quality.
Modern image forming devices are capable of producing pixels as small as 1/600th of an inch. However, image enhancement techniques can modulate the laser at the sub-pixel level. Many techniques are capable of modulating the laser at 1/64th of a pixel—producing a dot as small as 1/38,400th of an inch. When modulating the laser at 1/38,400th of an inch the minor changes in the photoconductor become significant—sometimes causing image enhancement techniques to degrade rather than improve the quality of printed output.